The Effect of Static Converters on Field Grading Materials ... · The Effect of Static Converters...
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The Effect of Static Converters on Field Grading Materials in Rotating Machines INTRODUCTION DISCUSSION Modeling ABSTRACT Silicon Carbide Microvaristors Some of objectives CONTACT Silicon carbide microvaristors are the most common type of stress grading materials being used for the design of coils and bars. the main objective of this project is determination of the microvaristor’s response over a wide range of electric field, frequency and temperature. Unfortunately, in spite of previous research in this area, an accurate measurement method, which clears the performance of these materials in mentioned conditions, has not been addressed effectively, owing to problems arising during practical measurement such as thermal run away and response determination in high frequency. On the other hand, such types of insulations are distinctively different from conventional insulations because of Maxwell-Wagner polarisation process, originating from composite mixture of the microvaristors and matrix of insulation. This project aims to provide a viable solution for above addressed issues. Two major challenges in this project are the nonlinearity and transient response of field grading materials. The transient response of linear insulation layers (including strand, mica and slot corona protection layer) can be found by frequency domain analysis and implementation of fast Fourier transform for data conversion to the time domain . However, for the nonlinear regime of insulation materials, such correlations are usually not available requiring consideration of permittivity harmonics as well. Furthermore, in order to obtain the fast response of stress grading layer, the responses of the other insulation layers are also required. With the introduction of transformer-less converters and their implementation in hydro- power generator systems, electric field distribution of insulation systems is drastically changed. Concerning consequential issues of converters, there are two important agents, namely voltage and temperature according to IEC standards for Type II insulation systems. These effects are summarized in Table I. Reza Sargazi Norwegian University of Science and Technology (NTNU) 5/27/2019 This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 764011. Insulation component Fundemental frequency Impulse repetition frequency peak to peak voltage of fundemental frequency peak to peak voltage of two consecutive impulses Impulse rise time Turn to turn insulation Mainwall insulation (Mica) Corona (CAT) and stress grading Layer (SG) Note: High Importance Low Importance Figure 1. insulation system in Roebel bars Voltage Current Voltage source Series Resistor Sample V HF and HV Transformer Amplifier V Matching Impedance Stator core Mica Copper SG Overlap CAT Traditionally, before the introduction of microvaristors, slot corona protection layer was used both in the slot as well as the end winding area for rotating machines above 4kV. With The advent of microvaristors in the 1970s, manufacturers could cut the corona protection layer just a few centimeters outside the slot without extension into the endwinding region. The resulting local electric field enhancment at this point was also reduced by introducing stress grading layer. this layer has a electric field dependent resistance reducing the high fields and pushing it out into the lower field areas. However, under impulse voltage stresses, its field controlling ability is influenced by factors summerised in table I. The insulation system of Roebel bars for rotating machines is shown in figure 1. Table I. different types of stress on insulation system fed by static converters Sample AC U I Voltage measurement Current measurement The effect of pressure finger on performance of SG The effect of mutual capacitance of bars on performacne of SG The effect of overlapp methods and resin insulation on SG layer Dielectric spectroscopy Dielectric spectroscopy of insulation is an effective method for insulation modeling. This method can be implemented both in time and frequency domain with possibility of data conversion between both domains in terms of linear dielectric materials. Generally, conductivity of dielectrics can be expressed as following equation. = 0 + ( ) ( 0 ( )+ ) Where E(t) is applied electric field, 0 is DC conductivity and P(t) is polarization process. General method for frequency domain spectroscopy is shown in below hierarchy.
Transcript of The Effect of Static Converters on Field Grading Materials ... · The Effect of Static Converters...
The Effect of Static Converters on Field Grading Materials in Rotating Machines
INTRODUCTION DISCUSSIONModeling
Figure 1. Label in 24pt Arial.
ABSTRACT
Silicon Carbide Microvaristors
Some of objectives
CONTACT
Silicon carbide microvaristors are the mostcommon type of stress grading materialsbeing used for the design of coils and bars.the main objective of this project isdetermination of the microvaristor’sresponse over a wide range of electric field,frequency and temperature. Unfortunately,in spite of previous research in this area, anaccurate measurement method, whichclears the performance of these materials inmentioned conditions, has not beenaddressed effectively, owing to problemsarising during practical measurement suchas thermal run away and responsedetermination in high frequency. On theother hand, such types of insulations aredistinctively different from conventionalinsulations because of Maxwell-Wagnerpolarisation process, originating fromcomposite mixture of the microvaristors andmatrix of insulation. This project aims toprovide a viable solution for aboveaddressed issues.
Two major challenges in this project are thenonlinearity and transient response of fieldgrading materials. The transient response oflinear insulation layers (including strand, micaand slot corona protection layer) can be foundby frequency domain analysis andimplementation of fast Fourier transform for dataconversion to the time domain . However, for thenonlinear regime of insulation materials, suchcorrelations are usually not available requiringconsideration of permittivity harmonics as well.Furthermore, in order to obtain the fast responseof stress grading layer, the responses of theother insulation layers are also required.
With the introduction of transformer-lessconverters and their implementation in hydro-power generator systems, electric fielddistribution of insulation systems is drasticallychanged. Concerning consequential issues ofconverters, there are two important agents,namely voltage and temperature according toIEC standards for Type II insulation systems.These effects are summarized in Table I.
Reza SargaziNorwegian University of Science and Technology (NTNU)5/27/2019
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 764011.
Insulationcomponent
Fundementalfrequency
Impulserepetitionfrequency
peak to peakvoltage of
fundementalfrequency
peak to peakvoltage of twoconsecutive
impulses
Impulse rise time
Turn to turn insulation
Mainwallinsulation
(Mica)
Corona (CAT) and stress
grading Layer(SG)
Note: High Importance Low Importance
Figure 1. insulation system in Roebel bars
VoltageCurrent
Voltage source
Series Resistor
Sample V
HF and HV Transformer
Amplifier V
Matching Impedance
Stator core
Mica
Copper
SGOverlapCAT
Traditionally, before the introduction ofmicrovaristors, slot corona protection layer wasused both in the slot as well as the end windingarea for rotating machines above 4kV. With Theadvent of microvaristors in the 1970s,manufacturers could cut the corona protectionlayer just a few centimeters outside the slot withoutextension into the endwinding region. The resultinglocal electric field enhancment at this point wasalso reduced by introducing stress grading layer.this layer has a electric field dependent resistancereducing the high fields and pushing it out into thelower field areas. However, under impulse voltagestresses, its field controlling ability is influenced byfactors summerised in table I. The insulationsystem of Roebel bars for rotating machines isshown in figure 1.
Table I. different types of stress on insulation system fed by static converters
SampleAC U I
Voltage measurement Current measurement
The effect of pressure finger on performance of SG
The effect of mutual capacitance of bars on performacne of SG
The effect of overlapp methods and resin insulation on SG layer
Dielectric spectroscopy
Dielectric spectroscopy of insulation is aneffective method for insulation modeling. Thismethod can be implemented both in time andfrequency domain with possibility of dataconversion between both domains in terms oflinear dielectric materials. Generally, conductivityof dielectrics can be expressed as followingequation.
𝐽𝐽 𝑡𝑡 = 𝜎𝜎0𝐸𝐸 𝑡𝑡 +𝜕𝜕
𝜕𝜕(𝑡𝑡)(𝜀𝜀0 𝐸𝐸(𝑡𝑡) + 𝑃𝑃 𝑡𝑡 )
Where E(t) is applied electric field, 𝜎𝜎0 is DCconductivity and P(t) is polarization process.General method for frequency domainspectroscopy is shown in below hierarchy.
